Well, here's my fix. Using this neat little program I downloaded called
Resource Hacker, I edited the buttons on the dialog box so that they would
be big enough to display the messages needed. Realizing that pinentry.exe
and pinentry-w32.exe were identical files (checking them in a hex editor
with file comparison function showed them to be exactly the same), I just
copied my edited version of pinentry.exe and renamed the copy as
pinentry-w32.exe. I have put both of them in a zip file called
pinentry.zip, and have attached this zip file to this email. Feel free to
distribute this on the official GPG4Win website. Note that the file name of
the attachment is "piz" not "zip", so before you extract its contents (for
use, or posting on your website) you will need to rename it from "piz" back
to "zip". I had to rename it from "zip" to "piz" because otherwise Gmail's
mail server scans inside the zip file and then for blocks it because it
detects exe files (and exe files are a format that can potentially harbor
malware). Even though this has no malware (as you can see by scanning it
with a virus scanner), Google's mail server takes extra precautions by
refusing to allow sending of executable files or even archive files that
contain executable files.

As far as I know, GPG4Win is a compiling/linking of GPG to be Windows
compatible, which means that the code was already altered to work with
Windows. Therefore native Windows code is already in use in the GPG4Win
variant of GPG. Therefore it should work correctly in every respect in
Windows (including correctly sized buttons).

Even so, this is a bug. As such, it should be fixed.